(1) Field of the Invention
The present invention relates to apparatus for cooling substrates, such as semiconductor wafers, glass substrates for photomasks, glass substrates for liquid crystal displays or substrates for optical disks, to a target temperature that is approximately room temperature, the substrates having been heated to a high temperature before and/or after one or more steps thereof during production.
(2) Description of the Related Art
Conventionally, this type of substrate cooling apparatus, by proportional-plus-integral-plus-derivative (PID) control or the like, maintains the cooling plate, which supports substrates during cooling thereof, at the same temperature as a target temperature to which substrates are to be cooled.
Substrate temperature falls rapidly during an initial cooling period but, owing to a control lag due to the heat capacity of the cooling plate, the cooling rate becomes lower as the substrate temperature approaches the target temperature. Thus, a long time is consumed in cooling the substrate, which results in the disadvantage that the apparatus has low throughput.
It has been a recent trend that, with a system having various process modules, a reduction in the number of cooling plates is required in order to reduce the size and cost of the system. However, this requirement has not been met because of the extended cooling time noted above.
Specifically, the conventional substrate cooling apparatus consumes about 45 seconds in treating one substrate, including its transportation into and out of the apparatus. Assume, for example, a system in which one such substrate cooling apparatus cooperates with first and second, photoresist applying apparatus each of which consumes 60 seconds in processing one substrate, and first, second and third developing apparatus each of which consumes 90 seconds in processing one substrate. In this case, as shown in the time chart of FIG. 7B, a waiting time occurs with each developing apparatus, and a long time is needed for completion of the developing process. This results in low processing efficiency. In order to improve the processing efficiency, two substrate cooling apparatus must be used. Consequently, the overall system will require a large installation space and here will increased cost. In FIG. 7B, "A" denotes substrates processed by the first applying apparatus, while "B" denotes substrates processed by the second applying apparatus, with the numerals showing the order in which the substrates are processed.